Could the fabric cause this?

When I was quilting this my machine would occaisionally throw a "knot" on the back of the quilt. Then when I was sewing the binding I experienced the same thing. I used two different machines. My Janome 6500 does the quilting on a frame and my Baby Lock was used for sewing the binding. Now I'm wondering if the fabric is the cause of this. The fabric is kind of stiff and definitely top quality.
Any thoughts?

I agree with cleaning the bobbin case, you'd be surprised how much lint can be found inside a machine. Also make sure your bobbin thread is feeding through the tension properly. I have known several ladies who have had this problem, and their bobbin wasn't threaded correctly [too loose]

I agree, but it happened on two different machines. That is puzzling me.

I know, it does seem odd but it could be a fluke that both machines are having the same problem at the same time. Maybe one Machine is having sympathy thread puke.
Did you use the same bobbin in both?
I spent an hour on test strips on my 6600 yesterday, it was skipping stitches on ZZ. I went through many needles(different sizes and types) and threads, along with rewinding bobbins and re threading 10,001 times. I could not get it correct and then it started to miss stitches on SS so I was done. It was for a pair of quick elbow length mittens for my 3yo so I just went ahead and did them. NOT ONE MISSED STITCH!! After I tried on the test fabric, the SAME poly fleece as the mittens, mess and missed stitches everywhere. How can that be explained?

A friend gave me a tip that I hadn't thought of before. Using canned air (like for a computer or keyboard) to clean out the bobbin area. I am fairly particular about the bobbin and keep it brushed out, but was having trouble with lint build-up. A puff or two of that canned air just made so much difference and far better than a brush has ever done. Now I keep it by my machine and hit it with a puff or two about every time I change the bobbin. Isn't it aggravating when the thread does that? Makes me stutter and stammer.....

I have read several places that canned air is bad for your sewing machine. It pushes the lint further into the machine and also contains moisture which is not good for your machine. You might ask your sewing machine tech about this.

I am wondering if this isn't an example of thread backlash. I just bought the Magic Genie Bobbin washers because sometimes I run into that w/frame quilting. Have had significantly less problems w/that since I adjusted my tension per this video http://www.youtube.com/watch?v=Q1mRhcquZTM

I was reading different things and some advice cleaning the bobbin area after each project, others were saying after X hours sewing, or even every time they replace the bobbin. I know I am always surprised at how much has built up in the bobbin case when I am sewing.

Mgmsrk....."sympathy thread puke"??? I was reading the link thinking how much I learn from this site as I read your post of sympathy thread puke and burst into laughter....too funny but what do I know, that may be a quilting phrase pasted down for centuries!

My Janome dealer told me to never use canned air on a machine that has a computer in it. You'll only blow
dust and moisture into the computer area. Not good for computers. I use a small brush to clean out the
dust in the bobbin area.

The only thing I use to clean my machine is a long pipe cleaner. It picks up the tiniest amount of lint. I've been complimented by my mechanic several time for how clean my machines are. Last time he was here he said my machines were the cleanest he's ever seen. Hence I only need to have them serviced every two years. I do clean them regularly but not at every bobbin change. He also said to give canned air a big miss.
This guy was head mechanic for Janome for 15 years here in Brisbane.

The teflon washers in the bobbin case sometimes helps. When I worked in a sewing factory, we always wound the thread twice around the guide when we were winding the bobbins to use. After the bobbins are wound, test the tread with your fingernail. If your nail sinks in easily or the wound thread seems a little spongy, it is wound too loosely. (spelling) The tread needs to feel a little firm. This can cause backlashing.